If there is a limit, it wasn't one I've ever come across but I've only made short notes on the Sony. I do believe there is a limit as to how much you can export (ie download from Sony back to PC) but that's not a feature I use.
I would either search the Sony forum here for more advice on it's note-taking ability or ask some questions there and hopefully someone will help:
https://www.mobileread.com/forums/forumdisplay.php?f=176